England secured the team gold as they reclaimed their status as world champions having previously won the title in 2017 in France.
The competition was held over two days in Split, Croatia and it also saw the boys team finish as runners-up in their competition.
Williams was named as captain of the girls team for the event, which attracted over 500 athletes from 29 countries.
And she won her individual 800m race to become the ISF World Schools 800m champion in style.
Leading from the gun, she set a fierce pace over the opening 400m with China’s Wan Fang and France’s Clara Pavlovic in hot pursuit.
Williams continued to drive the pace on the back straight and the competition were unable to hang on to her as she pulled away to win comfortably by over 3 seconds in 2:08.86 and secure 90 points for the girls team, one of the highest points contributions from all the events.
Day two saw Williams selected to run the 400m anchor leg of the medley relay (100m, 200m, 300m and 400m).
The scene was set with England leading the French by a small margin in the team competition and the medley relay being the deciding race.
After Jenna Blundell (100m) and Abigail Pawlett (200m) had run the first two legs, England and France were neck-and-neck and then Jasmine Jolly ran a terrific 300m leg to stay in contention.
Williams was handed the baton and settled in just behind the French while the Brazilian team were now leading.
But Williams, who secured her seventh international vest during the competition, attacked in the final 100m and reeled in the French athlete to take the team gold with a hugely impressive split of 56.7, nearly two seconds quicker than her 400m personal best.
Her club coach Shane Smith said: “Emily has once again performed brilliantly when asked to represent her country at a World Championships, producing a top quality time to win the 800m and then running a huge 400m PB to storm past the main competition in the anchor leg of the medley relay.
“I am very proud of her achievements, and especially that she was chosen to be the girls team captain.”
Williams is now hoping to achieve a European Under-20 qualification time for either the 800m or 1500m for the Championships, which are being held in Sweden in July.